Running weak

If it has twin carbs, adjustment or a blockage in the fuel supply ,would be my first check.

If it has twin carbs first adjust any slack out of the leftside cable. This should get the slides close to synchronised, and a noticeable difference should be obvious. If carbs already in balance or only have one carb. fitted then check HT lead, spark plug, plug cap and tappet clearances. Also fuel blockage as Robert suggests,

Simon.

Had the same problem with my Mk3 850 when it came back from a professional rebuild after 30 odd years sterling service in East Africa. Rebuild was otherwise an excellent job (should have been for what I paid !)but the left cylinder was running hotter than the right with corresponding bluing of the pipe out of the exhaust port while the right hand pipe only slightly yellowed and its silencer was noticable cooler than the left after a run. Left spark plug colour was light greyish brownindicating mixture on the weak side. Amal carbs were new on the rebuilt bike.

Engine started first or second kick andwas runningfine with clean acceleration and no flat spots ,but have nowraised the needle in the left carb to its richest setting (circlip in bottom groove) - was set in the top groove = weakest setting. Checked right carb which was also set in the weakest settingandleft itthere as plug colour was fine.

Hasn't made any noticable difference to engine performance but left silencer seems cooler than before straight after a runalthough still hotter than the right.

With the twin carb set up you've essentially got two separate engines whichrequireseparate tuning.

Alright James,

fair enough, both cylinders are never going to be exactly the same but I wouldn't expect to have such a difference on the carb. settings. Needles, jets, slides etc should be same position/size. I think your adjustments are treating the symptoms and not the cause. I would double check slides are synchronised, after that a leak down or compression test even though the engines been rebuilt. I never assume anything is as it should be.

If one cylinder is running hotter then it's burning more fuel or ignition timing on each cylinder is possibly different (with points set up),

Simon.

Previously simon_ratcliff wrote:

Alright James,

fair enough, both cylinders are never going to be exactly the same but I wouldn't expect to have such a difference on the carb. settings. Needles, jets, slides etc should be same position/size. I think your adjustments are treating the symptoms and not the cause. I would double check slides are synchronised, after that a leak down or compression test even though the engines been rebuilt. I never assume anything is as it should be.

If one cylinder is running hotter then it's burning more fuel or ignition timing on each cylinder is possibly different (with points set up),

Simon.

Indeed , all valid points. I've got Boyer ignition on my Mk 3. Carb slides are synchronised. Haven't made a compression test yet as bike otherwise runs perfectly with clean acceleration right through the rev range and an even tickover (after some minor pilot air screw and throttle stop fettling).

Atthe recent NOC event at Brooklands in June I wandered round checkingthe numerous Commandos there and 75% of them had blued pipes , either one or both , soclearly a common feature.

Hi James,

with both cylinders running evenly through the rev range both pipes would be subject to the same temperatures and therefore the pipes should be a very similar colour. The only time I've experienced different degrees of blue on the header pipes is when the carbs where out of balance. Any pair of exhausts displaying different colours would suggest something is wrong - I wouldn't call it a feature. An even tickover dosen't necessarily mean the slides are synchronised further up the bore,

Simon.